I'm sure you'll get a variety of responses on this one. And everybody seems to have their own take on it.
For ME... I won't date somebody who is married and is not in an open relationship, in that his wife knows he's seeing ME, and hopefully I would talk to her before anything happened to make sure she's REALLY on the same page.
In my opinion, someone who is cheating on their spouse is being dishonest, disrespectful and selfish. These are qualities they are expressing to the one person that supposedly means so much to them. Why would I think they would not at some point express those same qualities to me? If someone is dishonest with their spouse, they are comfortable being dishonest and secretive and I can only expect they will be the same way with me and that's not something I want or need in my life.
The whole "well he lies to HER, but not to me", or "he cheated on HER, but won't on me" pattern that sometimes women (not implying YOU) get into is just nonsensical.
That's the selfish reason. The other reason is that I just don't think I can separate myself from being a part of somebody else's humiliation, misery, suffering and drama, if I'm playing an active part in the scenario that creates that. No, it's not up to you to handle his relationship with his wife, but if you knowingly go into that relationship KNOWING she is being lied to, cheated on and disrespected you ARE playing a part.
Some people have no issues with that, or are better able to separate their responsibilities from others'. I can't.
And lastly... it will blow up on him at some point, and when it does, how much of the blowback do you want to be caught up in?
Again... just my opinion on the matter...